The present invention generally relates to control pulse recording circuits for magnetic recording and reproducing apparatuses, and more particularly to a control pulse recording circuit for re-recording (re-writing) control pulses by varying the duty cycle thereof depending on an arbitrary information while reproducing the square wave control pulses which are pre-recorded on a control track of a magnetic tape with a constant period for the purpose of carrying out a servo operation at the time of a reproduction in a helical scan type magnetic recording and reproducing apparatus.
A helical scan type magnetic recording and reproducing apparatus (hereinafter simply referred to as a video tape recorder or VTR) records and reproduces an information signal on and from a magnetic tape. A control track is formed along the longitudinal direction of the tape, and for example, control pulses having a period of one frame is recorded on the control track of the tape by a control head. Video tracks formed obliquely to the longitudinal direction of the tape are formed in a central portion of the tape, and the information signal such as a video signal is recorded on the video tracks by rotary heads. In addition, an audio track is formed at the upper end of the tape along the longitudinal direction thereof.
A pre-recorded tape is accommodated within a cassette case, for example, and is loaded into a main body of the VTR. During ordinary operation modes (for example, normal reproduction mode and variable-speed reproduction modes other than still picture reproduction mode) of the VTR, the tape is transported in a loaded state where the tape is in such a predetermined tape path that the tape is drawn out of the cassette case and is wrapped obliquely around the outer peripheral surface of a rotary body (for example, a rotary drum) for a predetermined angular range. This rotary body is mounted with the rotary heads. The tape in this predetermined tape path is transported by a capstan and a pinch roller which pinch and drive the tape. As is well known, the control pulses reproduced from the moving tape is used as a reference signal for a phase control system of a head servo circuit which controls the rotational phase of the rotary heads constant or as a comparison signal for a phase control system of a capstan servo circuit which controls the rotational phase of the capstan constant.
The control pulses at the time of the recording have the form of a square wave signal. But due to the differentiating characteristic and the like of the control head, the reproduced control pulses have the form of a positive polarity pulse at the rising edge of the pre-recorded square wave signal (pre-recorded control pulses) and the form of a negative polarity pulse at the falling edge of the pre-recorded square wave signal. Out of the positive and negative polarity pulses of the reproduced control pulses, only the positive polarity pulses are used as the control pulses in each of the servo circuits described above, and the negative polarity pulses are not used in the servo circuits. For this reason, it is possible to record a new information (for example, the title of the recorded information, the recorded date and other data) by varying the duty cycle of the control pulses at the time of the recording as will be described later on in the present specification, and a random access or the like can be performed by reproducing such new information at the time of the reproduction.
In this case, when playing the pre-recorded tape, it is necessary to perform a recording so as to vary the duty cycle of the control pulses while performing a reproduction so that the positive polarity pulses (hereinafter referred to as reference pulses) of the reproduced control pulses are supplied to the servo circuit to ensure a normal servo operation. Therefore, it is necessary to erase an interval between two mutually adjacent reference pulses and also re-record (re-write) the negative polarity pulses of the control pulses. The re-writing is performed by first reproducing the control pulse from the control track of the pre-recorded tape and obtaining a recording voltage. The recording voltage is a positive D.C. voltage from a first position immediately after a certain reference pulse to a second position (this second position changes depending on the duty cycle with which the re-writing is to be performed) immediately before a negative polarity pulse which is immediately after the certain reference pulse, for example. The recording voltage is a negative D.C. voltage from the second position to a third position immediately before the next reference pulse, and is zero from the third position to a position immediately after this next reference pulse. The control head is supplied with the recording voltage and is excited to the same magnetic pole as before in the interval between the first and second positions, excited to the opposite magnetic pole at the second position, and is continuously excited to this opposite magnetic pole to the third position. As a result, the control track is erased in the interval between the first and third positions, and the magnetic polarity is re-written between the second and third positions. The re-writing operation is ended at the third position.
After the re-writing operation is ended, the certain reference pulse is reproduced with the same timing as the original reference pulse with the constant period, and the negative polarity pulse immediately after the certain reference pulse is reproduced with a new timing. In other words, the control pulses are reproduced with the duty cycle thereof varied.
However, in the waveform of the re-written reproduced control pulses, noise is frequently generated at the first position where the re-writing is started and at the third position where the re-writing is ended. Hence, there are problems in that the noise may cause disorder in the synchronism, incorrect count of an absolute address and the like.
On the other hand, the recording voltage at the input of the control head is in the order of several volts, while the reproduced output at the output of the control head is in the order of several millivolts and is extremely small. However, the mode of the VTR is conventionally switched from the recording mode to the reproducing mode immediately after the re-writing operation is ended at the third position. For this reason, the recording voltage is still not converged to zero at the time immediately after the mode is switched to the reproducing mode due to the counter electromotive force or the exciting current at the time of the recording, and there is a problem in that it is difficult to reproduce the reference pulses.